3 Answers
3

Being an open relay means that you are accepting mail from unauthenticated users for domains that are not the domains you manage.

Postfix has some very simple config options to help:
mynetworks and *mynetworks_style* between them dictate from which network addresses messages to any destination will be accepted.
mydestination is used to specify which domains are domains that you manage (ie that you should accept messages for because they're yours).

I believe you'll need to set the mynetworks_style, mynetworks, and relay_domains settings in the Postfix configuration file. Basically the first two control which servers Postfix will accept mail from, and the last one controls which servers it will forward mail to. The exact settings depend on how you want your mail server configured (i.e. who is authorized to send emails through the server), but in general relay_domains should be set to only the hostnames of the mail server itself (same as mydestinations) and mynetworks_style can be set to host which means only accept emails from the local host itself.